Patent GB-1217894-A (GB)
Title: Switching circuit for electrical power conversion
Inventor: DEWEY CLYDE GILMAN
IPC: H02M7/757
Filed: 1968-03-19
Abstract: 1,217,894. Switching circuits. GENERAL ELECTRIC CO. 19 March, 1968 [24 May, 1967], No. 13297/68. Heading H3T. [Also in Division H2] In a switching circuit, for an A.C. power system, including a saturable core inductor 45 in series with a series of thyristor sections 41-44, the saturable inductor 45 is shunted by a resistance-inductance circuit R1, 61, 60 and a non- linear circuit 55-58 operative after the magnitude of current flowing through the resistanceinductance circuit attains a peak for providing a path across that part of the resistance-inductance circuit including the inductance 60 and a predetermined portion 61 of the resistance so that electric energy stored in the inductance 60 is dissipated in the portion 61 of the resistance. The total resistance of the resistance-inductance circuit has an under damping value and the resistance of the resistance-inductance circuit less that of the portion 61, has an over damping value with regard to the current build up in the saturable inductor 45 following turn on of the switching circuit. Turn on.-Shortly after turn on of the thyristors 41-44 (at t 1 , Fig. 6c, not shown), the voltage across inductor 60 is of opposite polarity and exceeds the voltage across resistor 61 so that the peak current is attained and the rectifier diodes 55-58 conduct and storage in 60 is dissipated. This is maintained until (t 6 ) when the diodes turn off and the inductor 45 finally saturates at (t 8 ). When each thyristor is switched on there is an oscillation caused by its inductive components and the stray capacitance of the connected power system but this oscillation is limited by the transient suppression circuit 45, 55-58, 60, 61 and R1 so that the forward current in the thyristors never fall below the holding value. As shown pairs of thyristor sections 41, 42 ; 43, 44 are connected to form a matrix and a plurality of matrices can be connected in series (Fig. 2, not shown) for use in a H.V., D.C. power transmission (Fig. 1). At each end of the D.C. transmission line 13 is provided a converter and rectifier comprising a bidirectional 3 phase double way bridge 20 connected to gererators and loads 11, 12 by circuit breakers 77. When the thyristor sections 41-44 are turned on before other similar matrices of the same series connection the terminal voltage 46, 47 is maintained across the saturable inductor 45 of the section turned on so that slower matrices are not subjected to excessive anode voltages. The voltages across the thyristors are equalized by R.C. dividers 51-54 and any voltage spike that might occur across which ever thyristor is last to turn on may be absorbed by and additional RC59. The inductor 45 subsequently saturates.  Turn off.-The diodes 55-58 prevent undesirable surges or transients at the terminals of the inductor 45 when the thyristors 41-44 are turning off. Near the end of commutation the magnitude of the current in the thyristors which are turning off decreases so that the inductor 45 saturates. The generation of a high voltage by the inductor 45 is avoided by the diodes 55-58 which are now forward biased so as to pass the current from the inductor 45. The current through the thyristor sections 41-44, proceeds to zero. and momentarily reverses to complete the turn off. Any energy stored in the inductor 45 after the thyristors turn-off is dissipated in resistors R1.  Each thyristor section 41-44 may-consist of parallel connected thyristors e.g. contained within a common housing and simultaneous triggering can be provided using light pulses. Heat dissipating arrangement may be provided for each thyristor. In equivalent circuits of the transient suppressing circuit 45, 55-58, 60, 61 and R1 (Fig. 7, not shown), an inductance (L) represents the commutating ; reactance constituted by half of the incoming and half of the outgoing phases.